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Identification of groundwater source rock in the southwestern part of Kuwait by using the mass balance technique
Abstract
The quality of groundwater and, hence, the uses of this water are a function of the interactions between the aquifer materials and the water passing through them (water-rock interaction). However, the concentrations of different ions in groundwater depend on the rock mineralogy through which the water passes and the speed of its migration along the flow path. In this study, the Hounslow (1995) simplified mass balance technique was used to deduce the source rock of the Kuwait Group aquifer water in the southwestern part of Kuwait. In addition, this technique was utilized to identify the major geochemical processes that influence the groundwater quality in the area. The results indicate that the source of Cl- and Na+ ions in the aquifer are from halite dissolution, where Cl- remains in the water and some Na+ ions are removed by reverse ion exchange processes due to the presence of clay minerals in the aquifer materials. The low concentration of SO2-4 in the aquifer indicates that there is no gypsum or anhydrite to dissolve or enough pyrite to produce SO2-4. The relationship between HCO-3 and SiO2 indicates that the SiO2 in this aquifer is due to silicate weathering, suggesting that the material of this aquifer could be hydrolysis-prone rock types, which are secondary products of the chemical breakdown of aluminosilicates such as feldspar. Finally, the ratio of Mg2+/Ca2+ indicates that carbonate minerals are unlikely to be present in this aquifer (except possibly as a cement for elastic sediments).
The Eocene Dammam carbonate aquifer in Kuwait is part of the most extensive limestone aquifer in the Arabian Gulf States. The principal artesian aquifer consists chiefly of 175-275 m thick, soft, porous chalky limestone and hard crystalline dolomitic limestone. The purpose of this paper is to describe the hydrochemistry of the groundwater and to identify the major geochemical processes operating within the carbonate aquifer during the last thirty years. A speciation model has been applied to the groundwater data to determine the chemical speciation during the studied period from 1972-2002 using the WATEQ4F program. Moreover, a mass-balance model has been applied for the period 1993-2002 to interpret the net geochemical reactions along the flow path. In addition, the WATEVAL program was used to deduce the source-rock and to identify the major geochemical processes that influence the groundwater quality of the Dammam carbonate aquifer. This study reveals that the main water genetic types are: SO4-Na, Cl-Mg and Cl-Ca, which represent both continental and marine environments as an origin of the formation of groundwater. The water chemical types are dominantly Na2SO4, CaSO4 and NaCl. Speciation calculations indicate that the groundwater is typically oversaturated with respect to calcite, aragonite and quartz, and it is undersaturated with respect to anhydrite, gypsum, halite and dolomite. The calculated mean values of Pco2 range between 2.77 × 10 -4 atm. and 8.72 × 10-3 atm. suggesting that the carbonate aquifer is a closed system with respect to CO2. The application of the mass-balance technique indicates that ion-exchange, reverse ion-exchange and dedolomitization are the prevailing geochemical processes in the aquifer. In addition, the ratio HCO3-/SiO2 > 10 suggests dominant carbonate weathering.
Al-Atraf is one of the water well fields of Kuwait supplying Kuwait City with the brackish groundwater obtained from the Kuwait Group aquifer of Miocene-Pleistocene age. The study determined the hydrogeological and hydrochemical characteristics of the groundwater in order to identify the major chemical processes that influence the groundwater quality of the study area. The results of the aquifer test analyses indicate that the Kuwait Group is a confined to semi-confined aquifer, with a transmissivity ranging between 62 and 321 m2/day. The flow net analysis implied that the groundwater moves from southwest to northeast. The estimated transmissivity values agree well with those calculated from the aquifer test. The results of the chemical analyses data of the Al-Atraf field show that the groundwater is mainly brackish, of NaCl and Na2SO4 water types. The groundwater is undersaturated with respect to halite, gypsum and anhydrite and supersaturated with respect to quartz, dolomite and calcite in the direction of groundwater flow. The average Pco2 of the groundwater is higher than the Pco2 of the earth's atmosphere indicating that the groundwater is supplied with CO2 during the infiltration processes and dissolves the carbonate minerals under closed-system conditions.